The modified slope-ratio technique developed on this project appears to be the method of choice for evaluating protein quality with young rats. There are indications, however, that these values may not be applicable in adult animals, and studies are required to determine whether the quality of proteins for adult animals are similar to those observed with young animals. Since man is a slowly growing animal, the values obtained with adult rats may be more applicable than those found with young animals. The amino acid composition of proteins is a primary determinant of protein quality, but it is as yet unclear how well biological estimates can be predicted from amino acid composition or what the best standard may be. Our work will be aimed at determining the most appropiate standard for the calculation of amino acid score and, depending upon how well the score predicts nutritive value, explore the reasons for the discrepancy. The relationship between energy intakes and protein utilization is still controversial. We will explore the available energy in various food sources and the effects of various degrees of energy and protein deficiency upon the ability of animals to maintain the protein content of various tissues. Finally, the question of whether the loss of tissue protein is achieved by modifications of the rates of synthesis, catabolism or both remains unresolved. It is our conclusion that essentially all the data currently available are suspect and, at best, repesent evaluations only of tissue proteins with very short half-lives and may not be representative of major protein reserves. Studies will be directed toward the effects of energy and protein deficiencies upon the metabolism of the major muscle protein, myosin.